Abstract: Austenitization temperature of the 1 400 MPa grade HSLA steel was measured by the Formastor-F Ⅱ thermal expansion measurement, and the effects of cooling time from 800 to 500 ℃ (t_8/5) on the microstructure and properties of the coarse grained heat affected zone (CGHAZ) of 1 400 MPa low alloy ultra-high strength steel was investigated by using Formastor-FⅡ, optical microscope (OM), Vickers hardness tester. The experimental results indicated that the critical point A_c1 is 710 ℃ and A_c3 is 820 ℃. With the increasing of t_8/5, the microstructure of CGHAZ changes from M to B + M, then to B, and finally to B + P + F. With the increasing of t_8/5, the hardness gradually decreases from 500 HV5 to 250 HV5, while the hardness value of the base metal ranges from 502 HV5 to 523 HV5. Therefore, under the condition of large welding heat input, the softening phenomenon of the 1 400 MPa grade HSLA steel is serious, and the welding heat input should be strictly controlled in the welding process.